In my last post, I suggested that we are probably on the road to a Supreme Court decision ruling that laws banning same-sex marriage are unconstitutional. But there are at least four different potential rationales the Court could adopt for such a ruling. Which one it chooses may have important implications for future cases. Here are the four possible strategies, and my thoughts on their potential effects:

In my view, the best rationale for striking down laws banning same-sex marriage is that they discriminate on the basis of sex, and are therefore subject to intermediate scrutiny under the Equal Protection Clause of the Fourteenth Amendment. I think this approach has strong support on precedential and originalist grounds. It also would create few slippery slope effects. For example, it could not be used to challenge laws banning polygamy. Polygamy bans discriminate on the basis of the number of people in a marriage rather than on their gender. The same goes for laws banning marriage between close relatives, such as those that exist in the 32 states that forbid or restrict cousin marriage. Such laws classify people based on family relationships rather than sex.

II. Laws Banning Gay Marriage are Subject to Heightened Scrutiny Because they Discriminate on the Basis of Sexual Orientation.

Courts could also strike down laws banning gay marriage under the Equal Protection Clause on the grounds that they discriminate on the basis of sexual orientation, and sexual orientation is a classification that requires heightened scrutiny, similar to that applied to gender or race. This approach was adopted by the Second Circuit in the Windsor case and the Ninth Circuit in a recent case ruling that gays and lesbians cannot be excluded from juries based on their sexual orientation. This reasoning would have implications beyond same-sex marriage because it would allow for similar challenges to any law that discriminates on the basis of sexual orientation. For example, a few states still restrict adoption by gays and lesbians in various ways. However, the impact of such a ruling may well be limited because laws that explicitly discriminate on the basis of sexual orientation are clearly on their way out. However, it’s possible that such a ruling could be used to challenge discrimination against gays and lesbians in hiring for government jobs. Such discrimination probably occurs in many cases where there is no explicit policy requiring it.

Like the sex discrimination rationale, this theory would not open the door to challenges to laws banning polygamy or cousin marriage, since such laws do not discriminate based on sexual orientation any more than they discriminate on the basis of gender.

III. Laws Banning Gay Marriage are Subject to Strict Scrutiny Because they Violate the “Fundamental Right” to Marriage Under the Due Process Clause of the Fourteenth Amendment.

Instead of relying on the Equal Protection Clause, the Supreme Court could instead rule that laws banning gay marriage violate the Due Process Clause. The Supreme Court has long interpreted that clause as protecting “fundamental rights” and has also ruled that marriage is one such right. Laws infringing on fundamental rights are subject to strict scrutiny, which is extremely difficult to survive. If the fundamental right to marriage is defined to include same-sex marriage as well as opposite sex, it would be an expansion of the Supreme Court’s standard for defining unenumerated fundamental rights, which holds that they are limited to those “deeply rooted in this Nation’s history and tradition.” If the right of marriage is broad enough to encompass same-sex marriage, litigants could also argue it is broad enough to encompass polygamous marriage or marriage between close relatives. If that right is broad enough to include one type of nontraditional marital relationship, why not others? Historically, cousin marriage and polygamous marriage have actually been more common than same-sex marriage. It is not entirely clear that such arguments would win, however. Much depends on the Court’s reasons for concluding that same-sex marriage falls within the scope of the “fundamental” right to marriage.

Several lower courts have ruled that laws banning same-sex marriage fail even the minimal rational basis scrutiny that applies to all legislation under the Equal Protection and Due Process Clauses if it is not part of a special category requiring heightened scrutiny. I think such arguments are unpersuasive for reasons outlined by Eugene Volokh. In theory, such an approach could ratchet up the level of scrutiny accorded virtually any law. In practice, however, the Court would probably try to limit the impact of this type of decision by holding that the type of rational basis scrutiny applied here is higher than usual because the law in question is motivated by “animus” against gays and lesbians. This would build on the reasoning of the Supreme Court’s decision striking down part of the Defense of Marriage Act in United States v. Windsor, which was in part based on the animus rationale.

Future decisions would then have to confront the question of what other laws might get higher-than-usual rationale basis scrutiny because they too are based on “animus.” Most other laws discriminating against gays and lesbians would surely qualify. Whether laws banning polygamy or cousin marriage qualify is less clear. Critics have pointed out that all sorts of laws are based in part on hostility to people who violate various societal norms. Even laws banning rape and murder are partly motivated by animus against rapists and murderers. Laws banning polygamy are at least in part the result of public hatred of polygamists. At least in the short term, courts would probably distinguish laws banning polygamy and cousin marriage from laws banning gay marriage, because the former have good policy rationales that are not based on unthinking prejudice. That may well be correct, but it underscores the extent to which the animus theory requires judges to make moral distinctions between different kinds of animus.

The Court could also adopt two or more of these theories simultaneously. When the same-sex marriage issue returns to the Supreme Court, the bottom line result will not be the only important issue at stake. If the Court strikes down laws banning same-sex marriage, much may depend on the reason why.

Ilya Somin is Professor of Law at George Mason University. His research focuses on constitutional law, property law, and popular political participation. He is the author of "The Grasping Hand: Kelo v. City of New London and the Limits of Eminent Domain" (forthcoming) and "Democracy and Political Ignorance: Why Smaller Government is Smarter."

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Comments our editors find particularly useful or relevant are displayed in Top Comments, as are comments by users with these badges: . Replies to those posts appear here, as well as posts by staff writers.